1. Technical Field
The present disclosure relates generally to patient ventilation systems, and more particularly to methods and systems for apnea and hypopnea identification for continuous positive airway pressure (CPAP) therapy.
2. Related Art
The respiration system of the human body provides needed oxygen intake, oxygen/carbon dioxide exchange, and carbon dioxide expulsion functions, each of which involves the lungs. In this regard, the lungs function as a gas-exchanging organ in which inhaled oxygen is passed to the blood, and collected carbon dioxide is passed from the blood to the air. Additionally, the lungs function as a respiratory pump that transports oxygen-rich air into the lungs, and the carbon dioxide-rich air out of the lungs.
There has been an increasing trend in the number of individuals suffering from obstructive sleep apnea and hypopnea, which interrupts and/or reduces respiration during sleep. Apnea is understood to refer to a condition in which there is a complete obstruction of the airway that results in the cessation of airflow for ten seconds or more. These respiratory pauses may occur repetitively, and alleviated only when the patient jolted back to a partially awake state. A partial obstruction leads to hypopnea, which is understood to be a reduction of peak flow into or out of the lungs by 30% or more from the peak flow baseline during normal breathing for a duration of ten seconds or more. The obstruction typically occurs in the tongue, the soft palate, or the upper airway. Although some degree of apnea and/or hypopnea is considered normal, in more severe cases, daytime sleepiness, fatigue, and impaired alertness may result as a consequence of reduced blood oxygen saturation and constant interruptions to sleep cycles. Chronic sleep apnea/hypopnea can lead to more serious health concerns, including cardiac arrhythmias, hypertension, and congestive heart failures with high mortality rates. Numerous cognitive functions are adversely affected as well.
Conventional treatments for sleep apnea may involve the prescription of one or more of a weight loss regimen (as apnea and hypopnea is closely correlated to obesity), medication, oral appliances, continuous positive airway pressure (CPAP) therapy, and less commonly, surgery. Generally, CPAP involves the application of positive pressure to open the patient's airway to prevent its collapse, as would otherwise occur during apnea. In a basic implementation, CPAP therapy applies a constant pressure that is not tied to the patient's normal breathing cycle. The positive airway pressure is desired in the inspiratory phase when the pressure differences between the lungs and the nose contribute to the collapse of the intermediate airway. However, supplying positive pressure flow into the patient during the expiratory phase generates resistance to the patient's breathing efforts, causing discomfort. Furthermore, toward the end of the patient's expiratory phase, flow and pressure in the airway is naturally minimal, such that positive pressure can cause additional discomfort. Notwithstanding the clinician's best efforts to prescribe a CPAP treatment flow rate that minimizes such extraneous pressure augmentation while ensuring the proper splinting of the airway during inspiration, the patient is still subject to higher pressures than needed throughout the breathing cycle. Accordingly, improvements that vary the delivered pressure according to breathing cycles or to patient efforts have also been developed.
The detection of abnormal breathing can be achieved with a variety of techniques. One involves measuring inspiration and expiration pressure over time, with running averages being compared to instantaneous measurements to determine whether any one cycle is outside a predetermined threshold. Information from multiple sensors, including nasal pressure, pulse oximetry, and plethysmograph sensors, is necessary to make accurate assessments. Instead of pressure measurements, systems such as those disclosed in U.S. Pat. No. 6,138,675 to Berthon-Jones are known to use flow measurements, though variances from a measured average over a time window are still relied upon to detect apnea.
Conventional CPAP devices that are intended for daily home use do not utilize an extensive number of sensors for cost reasons, and accordingly control is often based upon a single flow signal derived from pressure measurements or an estimated flow signal derived from a mathematical model corresponding to the operational parameters of the ventilation source, i.e., the blower motor. Accordingly, there is a need in the art for detecting apnea and hypopnea conditions with the single flow sensor measurement.